niu: improve ethtool support for gigabit copper cards

Introduced support for link speed and duplex setting (ethtool -s),
link advertising parameters and autonegotiation (ethtool -r):
- struct niu_link_config: split advertising and autoneg
  fields into active and target values (similar to speed
  and duplex fields)
- mii_init_common(): rewrite function to actually apply
  requested niu_link_config parameters instead of providing
  default initialization
- link_status_1g(): move parsing of MII registers into
  new link_status_mii() function (link_status_1g_rgmii()
  could possibly use this new implementation too)
- introduce simple nway_reset method
- fix incorrect XMAC_CONFIG_MODE selection for 10Mbps case

Signed-off-by: Constantin Baranov <baranov@mercdev.com>
Signed-off-by: David S. Miller <davem@davemloft.net>